1. Field of the Invention
The present invention relates to a liquid crystal display element used for OA devices and the like, and more particularly to a liquid crystal display device element of the reflection type which presents display by utilizing external light.
2. Description of the Related Art
Recently, with development of information technology and with awareness of the environmental issue, there is an increasing demand of developing display devices which is operable at low power consumption and with good visibility. The liquid crystal display (LCD) element of the reflection type using a cholesteric liquid crystal material is capable of performing display by using only external light. Therefore, there is no need of using the electric power for back light, and the power consumption by the LCD device is lessened. In this respect, the LCD element is suitable for the display of portable information device. The market places its hopes on further development of the LCD element. In particular, the reflection type LCD element using a memory-type liquid crystal elements is capable of sustaining the display once written without the help of the power source, except the drive power necessary at the rewriting time. Accordingly, there is no need of using the refreshing electric power, and hence a further reduction of the power consumption is corresponding achieved. In the reflection type LCD device using the non-memory type liquid crystal elements, when it is driven in a simple matrix driving manner, the inter-pixel crosstalk comes into problem. In this type of the LCD device, there is limited in the number of scanning electrodes. To avoid such a limitation, there is an approach to use active elements, such as thin film transistors, thin film diodes, or MIM (metal-insulator-metal) elements. However, the approach suffers from other problems; refreshing power consumption is inevitable, and the cost to manufacture increases as the result of using those active elements. On the other hand, the memory-type liquid crystal display element is free from the crosstalk problem, and hence, the cost increase problem resulting from the use of active elements. Further, this type of LCD device has an advantage of manufacturing liquid crystal display elements of large display capacity at low cost.
A bistable twisted nematic (BTN) display element is known as a variation of the memory-type liquid crystal display element.
Japanese unexamined patent publication No.Hei 11-344730 discloses a reflection type liquid crystal display based on the BTN system.
FIG. 1 is a sectional view showing a structure of a BTN type liquid crystal display element.
A liquid-crystal display element 11 shown in FIG. 1 includes a planar liquid crystal layer 3, and a transparent obverse surface electrode 2a formed on a substrate 1a and a light reflecting reverse electrode 2b formed on a substrate 1b. Those electrodes support the obverse side and the reverse side of the liquid crystal layer 3, respectively. An alignment layer 4a is layered between the obverse surface electrode 2a and the liquid crystal layer 3, and an alignment layer 4b is layered between the reverse electrode 2b and the liquid crystal layer 3. Those alignment layers have been subjected to a rubbing treatment to be in anti-parallel orientation. A circularly polarizing plate 5 including a ¼ wavelength plate and a linear polarizer is provided on the obverse side of the substrate 1a. A power source 20 is inserted between the obverse surface electrode 2a and the reverse electrode 2b, and applies a predetermined voltage to between the obverse surface electrode 2a and the reverse electrode 2b. 
The liquid crystal layer 3 is made of cholesteric liquid crystal having a helical structure, and a ratio d/p is set at 0.5 (where d : thickness of the liquid crystal layer 3, and p is a helical pitch of the liquid crystal). The twisted orientation of 180° A is stable for the initial orientation of liquid crystal molecules. A high voltage is applied from the power source 20 to those electrodes to reset the liquid crystal molecules such that the major axis of the liquid crystal molecules is oriented to be perpendicular to the substrates 1a and 1b. Then, a predetermined select voltage is applied from the power source 20 to the electrodes. As a result, bistable states, a twisted orientation state of 0° A or a twisted orientation state of 360° B, appear depending on the select voltage. The display can be executed by making use of the two orientation states A and B.
When the select voltage is lower than a threshold voltage, the orientation state of the liquid crystal molecules is changed to the twisted orientation state of 360° B having no birefringence effect, by the backflow effect. When the select voltage is higher than the threshold voltage, the orientation of the liquid crystal molecules is changed to the twisted orientation state of 0° A having the birefringence effect.
In the twisted orientation state of 360° B, if the product (retardation) of multiplying the thickness “d” of the liquid crystal layer 3 and the birefringence anisotropy is set to be smaller than the wavelength of light, Maugin condition is not satisfied. The liquid crystal layer may be handled as an optically isotropic layer. Accordingly, incident light 1 coming from exterior passes through the circularly polarizing plate 5 to be circularly polarized light C1. The circularly polarized light is reflected, by the reverse electrode 2b as by a mirror, into a reversely turned circularly polarized light C2. The reflected light C2 by the reverse electrode 2b is absorbed by the circularly polarizing plate 5 to present dark display. In the twisted orientation state of 0° A, if the retardation of the liquid crystal layer 3 is ¼λ, the external incident light I passes through the circularly polarizing plate 5 to be circularly polarized light C1. When it passes through the liquid crystal layer, it is transformed into linearly polarized light S. The linearly polarized light S reflected by the reverse electrode 2b passes through the liquid crystal layer again and is transformed into circularly polarized light C1. The circularly polarized light passes through the circularly polarizing plate 5 to present bright display.
The bistable states utilized by the BTN system are sustained for about several hundred ms. For this reason, it is impossible to sustain the image for a long time in a non-discharging state.
Another variation of the memory-type liquid crystal display element is a selective reflection type liquid crystal display element, which utilizes the selective reflection of cholesteric liquid crystal.
FIG. 2 is a cross sectional view showing a structure of a liquid crystal display element having the selective reflection function.
A liquid-crystal display element 12 shown in FIG. 2 includes a liquid crystal layer 3 made of cholesteric liquid crystal, and a transparent obverse surface electrode 2a formed on a substrate 1a and a reverse electrode 2b formed on a substrate 1b. Those electrodes support the obverse side and the reverse side of the liquid crystal layer 3, respectively. The liquid-crystal display element further includes a light absorbing layer 7 for absorbing light, which is formed on the rear side of the substrate 1b. An alignment layer 4a is layered between the obverse surface electrode 2a and the liquid crystal layer 3, and an alignment layer 4b is layered between the reverse electrode 2b and the liquid crystal layer 3. A power source 20 is inserted between the obverse surface electrode 2a and the reverse electrode 2b, and applies a predetermined voltage to between the obverse surface electrode 2a and the reverse electrode 2b. 
The alignment layers 4a and 4b in the liquid-crystal display element 12 are provided for uniformizing the display, and improving the reflectivity at the time of bright display and contrast. If the alignment layers formed by uniaxial orientation processing the alignment layers 4a and 4b by a rubbing treatment or the like are used, the memory property based on the liquid crystal layer 3 is lost or the contrast of the display lowers. For this reason, usually, there is no case that the alignment layers having undergone the uniaxial orientation processing are used for both sides of the liquid crystal layer 3.
In the liquid crystal display element of the BTN type, the helical pitch of the liquid crystal is several μm to several tens μm. In the liquid crystal display element of the selective reflection type, the helical pitch ranges from 0.2 μm to 0.5 μm. Further, a ratio of the thickness “d” of the liquid crystal layer to the helical pitch “p” (i.e., d/P) must be selected to be within a range from approximately 5 to 20. If d/p is smaller than 5, the reflectivity at the bright display time decreases. If it is larger than 20, the drive voltage must be increased. This results in remarkable increase of cost of constructing the drive circuit.
The cholesteric liquid crystal has a nature, called selective reflection, that it makes a kind of bragg reflection of a circularly polarized light whose wavelength is equal to the helical pitch “p” and has the same rotational direction as that of the helix. Accordingly, if the helical pitch “p” is selected to be equal to a specific wavelength within the visible wavelength region, the cholesteric liquid crystal reflects light of the same color as that of the specific wavelength.
In the figure, the cholesteric liquid crystal exhibits bistable states, a planar orientation (P orientation) P in which the helical axis of the liquid crystal molecule is substantially parallel to the normal line of the substrate, and a focal conic orientation (F orientation) F in which the helical axis of the liquid crystal molecule is vertical to the normal line of the substrate. In the P orientation, light I incident on the liquid crystal layer 3 is selectively reflected and color is visually presented. In the focal conic orientation F, the incident light I passes through the liquid crystal layer, so that if the light absorbing layer 7 on the reverse side of the substrate 1b colored black, black color is visually presented. In this case, for the switching of the P orientation to and from the orientation F, high voltage is first applied to the display element so that the liquid crystal molecules are orientated in the direction vertical to the substrate (the liquid crystal molecules are reset), and then a predetermined select voltage is applied to the display element. In turn, the P orientation or the focal conic orientation F is set up depending on the applied select voltage.
The switching procedure of the display element under discussion is similar to that of the BTN type display element shown in FIG. 1, but is different from the latter in mechanism.
When the select voltage is lower than a predetermined threshold voltage, the molecules of the cholesteric liquid crystal are orientated to have the P planar orientation which is topologically continuous to the perpendicular orientation state set up by application of the high voltage. The change to the P orientation occurs irrespective of presence or absence of the anchoring at the boundary between the alignment layers 4a and 4b of the liquid crystal layer 3. This is different from the P orientation of the liquid crystal molecule caused by the backflow effect dependent on a specific anchoring. When the select voltage is higher than the threshold voltage, the liquid crystal resumes the helical structure, and the rotational torque caused by the positive dielectric anisotropy acts and the helical axis rotates, so that the orientation of the liquid crystal molecule changes to the F orientation.
In the liquid crystal display of the selective reflection type, the bistable states of the P orientation and F orientation are permanently sustained, and the long-time holding of the image is possible. Since the polarizing plate is not required, the display is bright. Since the color filter is not used, the display element presents bright colored display. The display element presents halftone display if the P orientation and the F orientation are stabilized in a mixed state. However, the black and white display is difficult since the display is colored with the color of a specific wavelength. Further, the d/p must be 10 or higher to secure a sufficient reflectivity. In this respect, the drive voltage must be high.
Accordingly, an object of the present invention is to provide a liquid crystal display element, which can visually present display in black and white, and sustain an image for a long time in a powerless state.